Volume limiting amplifier



y 24, 1951 J. L. MERRILL, JR, ET AL 2,561,747

VOLUME LIMITING AMPLIFIER.

Filed Sept. 18, 1945 CHARAC TER/S TIC FROM $09 BOT/1' NETWORKS ACT/N6Cl-lARACTfR/ST/C FROM NV OUTPUT By M A? ore/w.

lA/VENTORS Patented July 24, 1951 VOLUME LIDHTING AMPLIFIER Josiah L.Merrill, Jr., Port Washington, N. Y., and George C. Reier, Westfield, N.J., assignors to Bell Telephone Laboratories, Incorporated, New York, N.Y., a corporation of New York Application September 18, 1945, Serial No.617,004

I. This invention relates to signal transmission systems andparticularly to the control of amplification in suchsystems by the useof volume limiters.

' One object of the invention is to provide a sub stantially constantspeech level output circuit in which the gain is varied automatically bya combination of volume limiters having non-linear elements such asthermistors to maintain the output level constant or withinpredetermined limits without introducing objectionable distortion of thesignals.

Another object of the invention is to provide a substantially constantspeech level output circuit which affords the required degree and speedof control, and the operation of which is more economical than that ofcomparable circuits.

A feature of the invention is the employment of non-linear elements suchas thermistors which are inherently slow acting, non-distorting devicesbut which, when operated in combination, interact to obtain'a relativelyfast attack time without the introduction of distortion.

Various automatic volume control systems have been proposed heretofore.In some of these a portion of the signal is rectified and used as avariable grid bias to control the gain of one or more amplifying tubesto compensate for changes in the input level. While systems of this typecan be devised in accordance with well-known circuit theory to meetalmost any particular requirements, they involve one or more additionalvacuum tubes and other circuit elements which often unduly increase thecost of the system. To provide a more economical system it has beenproposed to employ a non-linear resistor in series with a negativefeedback path around the amplifier, or in some cases in shunt to thetransmission path. There are two well-known types of non-linearresistors; namely, thermistors which vary in resistance relativelyslowly in accordance with the heating efiect of the current andvaristors which vary in resistance instantaneously iii-accordance withthe magnitude of the current. If the action of the thermistor is toofast it may follow the syllabic variations of speech. .Such action isundesirable in a volume limiter as it introduces distortion. Therefore,a relatively slow thermistor element necessarily must be. used. However,.such an element reacts too slowly to large, sudden changes in speechvolume level to be entirely satisfactory when used alone. On the otherhand while a varistor acts instantaneously, it introduces objectionabledistortion by suppressing or chopping off the individual signal peaks.

1 Claim. (Cl. 179171) According to this invention, fast acting automaticvolume control is obtained in a very in expensive manner and without theintroduction of distortion by the use, in combination, of two relativelyslow acting non -linear networks. 'llhese networks may in their simplestform consist of two thermistors connected as series elementsin negativefeedback connections of two tandem amplifier stages, or two thermistorsconnected as simple shunts to the input and output circuits of a fixedgain amplifier. In either case the reduction in the attack time obtainedis believed:

to be brought about by the over driving of the second thermistorproduced by the thermal lag in the first thermistor which is in a lowerlevel por-v rectly in parallel provided one thermistor is of greatersensitivity than the other; that is, one

thermistor will react at lower signal voltage.

In the associated drawing Fig. 1 is a limiting system according to theinvention. Figs. 2, 3, 4

and 5 are diagrams illustrating the operation of the thermistornetworksaccording to the invention. Fig. 6 is a two-stage feedback amplifieradapted for operation as a fast operating limiter. Fig. 7 is a fixedgain amplifier adapted for operation as a fast operating limiter.

Referring to Fig. l of the drawing, signals from the source I aretransmitted to the load ZL through the networks A and B which maintainthe voltage across the load either constant or within a certain range ofvalues as required. Networks A and B are volume limiters employingthermistors and having input-output characteristics shown in Figs. 2 and3, respectively.

As shown in Figs. 2 and 3 these networks provide at low input levels acertain amount of amplification. Network A provides a gain equal to theline .()A and network B provides a gain rep resented by line OH. It isnot a requirement that amplification be provided. However, ifamplification. is not provided and We have an entirely passive circuit,it is essential that one network be of greater sensitivity than theother; that. is one network will react at lower signal voltage. Inprinciple, the networks may be of different sen sitivity, or if they areof the samesensitivity, they must be connected to the circuit at pointsof different signal level.

Referring to Figs. 2, 3, 4 and 5 of the drawing, the input and output ofnetworks A and B are expressed in decibels. The input to network A istherefore designated 20 log 0 and the output is designated V0 where V0is an arbitrary reference voltage, V1 is the input voltage and V2 theoutput voltage. In network B, V2 is the input voltage and V3 the out putvoltage.

20 log Network A is shown as limiting along the line CD while network Bis shown as limiting along the line MK. The maximum steady state voltageoutput of network A is represented by OC.

Let us assume that at the time O a sudden large input voltage OF isapplied to network A. At the time 0, network A will not be in thelimiting condition and the voltage output will therefore be OE. VoltageOE will be applied. to the input of network B which also will not be inthe limiting condition and will therefore have a voltage output OS. Ifthe input voltage OE applied to netever, the input voltage to network Bis not constant but is decreasing. At the time 0, the output voltage ofnetwork B as shown in Fig. 5 is not only decreasing at the rate therateof change of the curve SZ, it is decreasing at a faster rate becauseV2 at the time 0 is also decreasing as shown by the curve EXD of Fig. 4.The voltage V3 is directly related to V2 by a factor ,lL so that V3 isequal to ive where a is the gain of network B. This gain, of course,changes with input voltage V2 in the limiting range of network B but atthe time 0 it will have a definite value. Likewise at other times near 0the rate of decrease of V3 will be greater than it would be if V2 wereconstant. However, as the r voltage V2 diminishes, a pointwill bereached where V3 will no longer decrease at a greater rate than. itwould have if V2 had been held constant. The curve that shows asimultaneous action of bothnetworks is designated SYZ on Fig. 5. Theaddition of network A to the system has decreased the attack time by YZas shown on Fig. 5.

The use of .more than one limiting network increases the limiting range.For example, the range of the system consisting of networks A and B willbe broader than the range of either network alone. There are two reasonsfor this: first, the power handling capacity of the system has beenincreased by the addition of more elements each capable of dissipating acertain amount of power; and second, the networks are designed to startlimiting at different levels. One network will account for the beginningportion of the limiting characteristic and the other will-act on thehigher input level. In regions where only one network is acting theattack time will not be as fast as it will be on the overlap portion ofthe characteristc where both network are limiting in the mannerdescribed above. Therefore, where the attack time is a majorconsideration the overlap portion of the range should be as broad aspossible. Where range alone need be considered the overlap can benarrow; one network acting after the other has completed its limiting.

Fig. 6 of the drawing shows a; particular structure of the networks Aand B. This is a simple type of feedback circuit well known in the artwith the thermistors T1 and T2 being the nonlinear elements providingthe limiting action. The constants are so related as to provide thedesired over-all input-output characteristic and a relatively fastattack time. It will be observed that the two stages of amplificationare not independent. of each other. The feedback voltage from the secondstage appears across the output circuit of the first stage. This in turnaffects the amount of feedback in the first stageand thereby afiects thelimiting action.

Fig. 7 of the drawing shows a combination. of volume limiters togetherwith a fixed gain amplifier to provide a fast operating circuit whichgives constant output level for speech input levels within apredetermined range. Networks A and B may be two identical thermistorsemployed to introduce loss into the circuit. The operation. of thicircuit is similar to that of the circuits shown in Figs. 1 and 6.

What is claimed is:

In a signal transmission system, a signal transmission line comprisinginput and output. terminals and adapted to receive and. transmit signalsthrough said terminals, a source of signals connected to said inputterminals, and volume limiting means operatively connected to. said linefor controllin the energy level of the signals from said sourceincluding an amplifier comprising at least two tandem stages ofamplification wherein each succeeding stage provides an energy level forsaid signals greater than that provided by the stage preceding it, andeach of said stages has an individual degenerative feedback circuit anda thermally sensitive resistor connected as a series element in saidfeedback circuit, said resistor having a specific resistance which isnot directly affected by the magnitude of the signals in said feedbackcircuit and cannot be caused to vary while the temperature of saidresistor remains constant but is caused to vary non-linearly andinversely as the temperature of said resistor varies under the thermalinfluence of the signal in said feedback circuit.

JOSIAH L. DERRILL, JR. GEORGE C. REIER.

REFERENCES CITED The following references are of record in the

